(a) Field of the Invention
The present invention relates to a power train of an automatic transmission. More particularly, the present invention relates to a power train of an automatic transmission that realizes seven forward speeds and two reverse speeds.
(b) Description of the Related Art
A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A power train of such an automatic transmission that includes the plurality of planetary gear sets changes rotating speed and torque received from a torque converter of the automatic transmission, and accordingly changes and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance.
For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a power train such as durability, efficiency in power transmission, and size depend a lot on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a power train are also under constant investigation.
A manual transmission that has too many speeds causes an inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations without needing any manual operation.
Recently, power trains of automatic transmissions that realize seven forward speeds have been developed. For example, U.S. Patent Laid-Open No. US2005/0113205 A1 discloses a power train of an automatic transmission that realizes seven forward speeds and one reverse speed by using four planetary gear sets, five brakes, and three clutches. However, since the power train of US2005/0113205 A1 includes four planetary gear sets, five brakes, and three clutches, the power train is heavy. In addition, it is difficult to dispose frictional elements such as brakes and clutches in the transmission according to the power train of US2005/0113205 A1.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.